One of the well-known mass-analyzing methods for identifying a substance having large molecular weight and for analyzing its structure is an MS/MS analysis (or tandem analysis). In a general MS/MS analysis, an ion having a specific mass-to-charge ratio is first selected as a precursor ion from a variety of ions originating from a sample. Next, the precursor ion is dissociated into a variety of product ions by a CID (collision induced dissociation) process or other process. The product ions thus created are mass-analyzed to create a mass spectrum (an MS/MS spectrum or MS2 spectrum) for example. Then the mass spectrum is analyzed to identify the original molecule and determine its structure. In some cases, an MSn analysis may be performed in which the following operation/analysis is repeatedly performed: a precursor ion is further selected from product ions to be dissociated and the product ions created by this operation are mass-analyzed.
Conventionally, a liquid chromatograph mass spectrometer (LC/MS) in which a mass spectrometer capable of such MSn analysis is used as a detector of a liquid chromatograph has been developed as disclosed in Non-Patent Document 1. In such an LC/MS, data having three dimensions, i.e. a retention time, mass-to-charge ratio, and signal intensity is collected. Based on the data, a mass chromatogram, mass spectrum, and total ion mass chromatogram are created. The mass chromatogram shows the relationship between the retention time and signal intensity for a specified mass-to-charge ratio, the mass spectrum shows the relationship between the mass-to-charge ratio and signal intensity for a specified retention time, and the total ion mass chromatogram shows the relationship between the retention time and signal intensity without a limitation of the mass-to-charge ratio. It is also possible, based on the data obtained from the analysis, to set an ion to be focused as a precursor ion to perform an MS2 analysis for this precursor ion.
Conventionally, in selecting a precursor ion based on the data obtained from an MS1 analysis, a selection reference is used such as: descending order of the signal intensity or ascending order of the mass-to-charge ratio of a plurality of peaks having a signal intensity larger than a predetermined threshold value among the peaks appearing in a mass spectrum. However, with such conventional precursor ion selection methods, an ion selected as a precursor ion is not always one originating from the component of interest in the case where many peaks appear for example.
Non-Patent Document 1: “LCMS-IT-TOF”, SHIMADZU NORTH AMERICA, Internet <http://www.ssi.shimadzu.com/products/product.cfm?product=lcms_it_tof> [Apr. 15, 2008]